Organic polymers to precede nano semi

With the advent of organic light-emitting diodes (OLEDs), organic semiconductors have thrown down the gauntlet as alternatives to silicon in some applications. But there remains a large hurdle-inexpensive "weather proofing" to ensure a long life.

This summer, Olight joined NuVue as brand names to conjure with. These new polymer-enabled organic LED displays come from DuPont (E.I. du Pont de Nemours and Co.) and Kodak, respectively. Lightbulb-making giant in Germany, Osram GmbH (Munich), also announced its brand name-Pictiva. Engineering evaluation kits are available.

"Yes, organic LEDs are already here, but I think it will be another three or four years before all the technical problems are solved in terms of a longer lifetime and better encapsulation to protect the displays from moisture and oxygen," said Salvo Coffa, director of silicon technology, Optoelectronics, Bioelectronics and Nanorganics at ST Microelectronics N.V. (Geneva).

Organic LEDs are more flexible, cost less and, yet, are more colorful because they emit light rather than just filter a backlight, like LCDs. This is how they work: Organic thin films are fabricated between two conductors so that when an electrical current is passed through them light is emitted in a process called electrophosphorescence. (The applied current drives electrons over a band-gap into a high-energy orbit, and when they fall back across the gap, the energy is released by emitting light in a specific band.)

OLEDs are very thin (500 nanometers) and, yet, produce self-luminous displays that can be fabricated atop a plastic or amorphous-silicon substrate similar to an overhead-projector transparency. They also have a wide viewing angle of 160 degrees and require only a 2-volt power supply. That's the good news.

The bad news is that, today, designers can only acquire small glass-encapsulated units, which are fine for small displays housed inside the body of a device (like a digital camera). Glass encapsulation mitigates contamination with oxygen and moisture, but the long-range promise of OLEDs is that the organic semiconductors will become so well understood that paper-thin displays so inexpensive as to be disposable can eventually be created.

Toward the nanoscaleOrganic semiconductors have been the subject of intense research worldwide. IBM Corp., for instance, claims to have attained performance comparable to amorphous silicon transistors using thin-film transistors based on pentacene. Application groups worldwide are currently perfecting applications of organic thin-film transistors for electronic-paper, ink-jet printed circuitry, organic displays and disposable "smart cards." Such circuitry will eventually be manufactured on inexpensive substrates manufactured with roll-to-roll processing.
Downsize nanoscaleDi-block copolymer templates-self-organizing thin films for masking nanometer-scale structures onto large silicon wafer areas-use a polymer to help downsize nanoscale patterns on silicon. Experimental fabrication processes based on these polymer templates include pattern transfer by reactive-ion etching, chemical etching and metal deposition. As a high-resolution patterning step in the fabrication of more complicated device structures, researchers are also experimenting with diblock copolymers to pattern semiconductor devices, spin-dependent electronic devices and magnetic media.

While laboratories worldwide struggle with transferring diblock polymers to silicon, others struggle with transferring organic semiconductors to a plastic substrate, where they can be used in circuits. Organic chemists are beginning to synthesize all the common components on the molecular scale, but in a beaker. Without transferring them to a substrate, they are just left floating around disconnected.

A prototype 15-inch flat-panel organic light-emitting diode (OLED) technology is but the first step for OLED-based televisions.

Recently, progress was reported by University of Chicago professor Luping Yu, who announced fabrication of the world's smallest diode-in-a-beaker-an organic device measuring just 2.5 nanometers. The operation of the polymer-based p-n junction diode was synthesized using organic chemistry.

Organic semiconductors also hold the promise of making memories with an organic molecule as the switching element-switching either electrically or optically-so that very dense arrays can store bits on individual molecules.

"For organic semiconductors, the big problem is architecture. Sure, you can write all this information on single molecules, but how are you going to access it without a massive interconnect?" asked ST's Coffa.

One solution popularized by IBM marries the nanoscale world of MEMS to that of organic polymers: a MEMS cantilever that polls a polymer-based memory with massive parallelism. IBM fabricated cantilevers with thousands of read/write "heads" that could poke holes to make "1s" and erase them to zero by melting the polymer back into the hole. Such "moving parts" inside an organic polymer chip work either by moving a substrate back and forth under a stationary read/write head array or by spinning a nanosized disk under the head array.

IBM Nobel laureate Gerd Beinnig said such nanotechnology techniques will eventually produce a "thousandfold increase in data storage density," potentially leading to petabit-sized devices.

"Someday, our prototypes may lead to replacement chips that you can plug into the same sockets as current flash memory chips, but with incredible storage capacity and only about 100-milliwatts power consumption," said Peter Vettiger, Millipede project leader with IBM Research (www.eetimes.com/story/OEG20020611S0018).

"We all agree that we can use nanotechnology to make these devices, but the real question is whether we can do so reliably and very cost-effectively-that remains to be seen," said Coffa.

Research for 'perfect' OLEDs is the quest at various industry and academic labs. Here is a working OLED array from the MIT lab.

Silicon here to stay
While organic semiconductors gain ground for low-cost, wide-array applications, no one is selling the 100-kHz devices as replacements for gigahertz-caliber silicon chips.

What's more, while polymer memories may someday be offered as plug-replacements for flash memory, nanotechnological advances promise to keep traditional silicon-based chips on top of the high-performance game.

In particular, carbon nanotubes-a new form of carbon that more closely resembles diamonds than pencil lead-enable electrons to enjoy "ballistic" transfer speeds. IBM, for instance, has demonstrated carbon-nanotube field-effect transistors (CNFETs) that outperform even the most optimistic predictions for next-generation silicon chips.

"CNFETs are not yet ready for commercialization, but our results indicate that they will outperform even the most advanced silicon transistor designs for future nanoelectronic applications," said Phaedon Avouris, manager of nanoscale science at IBM Research (www.electronicstimes.com/story/OEG20020520S0020).

Likewise, future nanoelectronic applications will likely use not only electrons for processing, but will also convert electricity into light for on-chip optical processing, too. To prove the point, IBM recently demonstrated CNFETs that emit light.

"These results show that nanotube-based light emitters can potentially be integrated with silicon electronic components, opening up new possibilities in electronics and optoelectronics," said Avouris.

By advancing understanding of the electrical properties of nanotubes that emit light, IBM hopes to create chips that convert electrons to photons and back again whenever convenient.

By carefully characterizing the band-gap properties of nanotubes, which enable them to emit light, IBM hopes to be able to produce and process optical information in the popular communications wavelengths.

"We are investigating the variable band-gap of semiconducting nanotubes, which depends upon the diameter of the nanotube. We are trying to get nanotubes of different diameters for different communications wavelengths . . . we still have issues to answer, because if you want to keep improving things, you have to understand the details for the long term. That's the only way to avoid surprises when you go to commercial production," said Avouris.